Thiourea at

Article pubs.acs.org/JPCB

Dissolution and Metastable Solution of Cellulose in NaOH/Thiourea at 8 °C for Construction of Nanofibers Zhiwei Jiang,† Yan Fang,† Yanping Ma,§ Maili Liu,‡ Ruigang Liu,§ Hongxia Guo,§ Ang Lu,*,† and Lina Zhang*,† †

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China § State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: To develop a facile approach for the dissolution of cellulose, a novel solvent (9.3 wt % NaOH/7.4 wt % thiourea aqueous solution) was used, for the first time, to dissolve cellulose within 5 min at 8 °C. The results of NMR and Raman spectra demonstrated that stable thiourea···OH− complexes were formed through strong hydrogen bonds in NaOH/thiourea at room temperature. Moreover, the strength of the hydrogen bonds in thiourea···OH− complexes was much higher than that in urea···OH− complexes, and the number of thiourea···OH− complexes increased significantly in 9.3 wt % NaOH/7.4 wt % thiourea compared to that in 9.5 wt % NaOH/4.5 wt % thiourea, which dissolved cellulose at −5 °C, leading to the dissolution of cellulose at a relatively high temperature (8 °C). The cellulose that dissolved at such a high temperature was metastable. The results of dynamic light scattering and transmission electron microscope experiments confirmed that the extended cellulose chains and their aggregates coexisted in the dilute cellulose solution. Interestingly, stiff cellulose chains could be self-assembled in parallel to form nanofibers in the metastable cellulose solution, from which cellulose microspheres consisting of nanofibers could be easily produced by inducing heating. This work not only proposed a novel method for the dissolution of cellulose in aqueous system at temperatures over 0 °C but also opened up a new pathway for the construction of nanofibrous cellulose materials.

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DMAc/LiCl,15 ZnCl2 hydrates,16,17 and ionic liquids.18,19 Recently, a series of novel solvent systems precooled from −12 to −5 °C with a preferable dissolving capacity for cellulose have been reported in our laboratory, such as NaOH/urea, NaOH/thiourea, and LiOH/urea aqueous solutions.20,21 The formation of new hydrogen bonds between cellulose, thiourea or urea, NaOH, and water played an important role in breaking the native close packing of the cellulose chains.22,23 The presence of urea or thiourea dramatically improves the dissolution of cellulose in NaOH and the stability of the resultant cellulose solution. It is worth emphasizing that the temperature of cellulose dissolution in those solvents must fall below zero. This could be explained by the fact that the decreasing temperature could enhance the hydrogen-bonding interactions between macromolecules and solvents in the cellulose solution to promote the cleavage of the cellulose chains through the formation of new hydrogen bonds.22,23 However, low temperatures could affect the industrial operation

INTRODUCTION The sustainable application of biomass1,2 should make it possible to meet the high worldwide demand for the petroleum and natural gas resources that are being depleted and overcome the increasingly serious global climate change. Lignocellulosic biomass has long been recognized as a potential sustainable source of biofuels, chemicals, and biomaterials.3−7 As the most prevalent of the three lignocellulosic polymers (cellulose, hemicellulose, and lignin), cellulose, as a linear poly(1→4)-βD-glucan chain, contributes 35−50% of the dry weight of biomass. However, cellulose is insoluble in a majority of organic solvents and exists mainly in crystalline microfibrils8 because of the presence of a strong inter- and intramolecular hydrogen bonding network,9 which hinders its application in many industrial areas. Thus, effective dissolution of cellulose is of great importance for the comprehensive utilization of cellulose, especially considering that the traditional viscose process is not favored because of the use of toxic CS2 and the consequent serious environmental pollution.10 Many solvent systems have been extensively developed for cellulose dissolution,11 including alkali aqueous solutions,12 Nmethylmorpholine-N-oxide/water (NMMO/H 2 O), 13,14 © 2017 American Chemical Society

Received: October 27, 2016 Revised: February 6, 2017 Published: February 7, 2017 1793

DOI: 10.1021/acs.jpcb.6b10829 J. Phys. Chem. B 2017, 121, 1793−1801

Article

The Journal of Physical Chemistry B

NaOH/7.4 wt % thiourea aqueous solution (mole ratio of NaOH/thiourea/H2O, 1:0.26:20), which is coded as Solvent-2; and 10 g of thiourea was added to 100 g of 10 wt % NaOH to achieve 110 g of 9.1 wt % NaOH/9.1 wt % thiourea aqueous solution (mole ratio of NaOH/thiourea/H2O, 1:0.26:20), which is coded as Solvent-3. Thiourea (5 wt %) and thiourea (8 wt %) aqueous solutions were coded as 5% thiourea and 8% thiourea, respectively. Cellulose could be dissolved in Solvent-1 precooled at −5 °C or in Solvent-2 precooled at 8 °C, which are coded as Solution-1 and Solution-2, respectively. Cellulose Solubility. Dry C-1 (0.1 g) was dispersed in 9.9 g of NaOH/thiourea solution and stirred for 1 h at 25 °C to measure the transmittance of the resultant cellulose solutions. The cellulose solution was then isolated by centrifuging at 7000 rpm for 20 min. The dissolved and insoluble cellulose fractions were neutralized with 1 M H2SO4, washed with water and acetone, respectively, and dried at 100 °C to a constant weight. The solubility (Sa) of cellulose was calculated as

and lead to a high energy consumption. Therefore, it is preferable for the cellulose dissolution to occur at a relatively higher temperature. Despite the similarity in their molecular structures, urea and thiourea form different crystal structures via hydrogen bonding. Urea is linked in a head-to-tail manner, whereas thiourea is linked by centrosymmetric cyclic H-bonds.24 The NH2 and CO(S) groups in urea (thiourea) should provide hydrogen bonding that is enough to induce conformational changes in DNA, nucleotides, and proteins.25,26 Furthermore, they are also capable of forming complexes with different coordination numbers with several metals27 and anions,28,29 and the substantial complex formation even takes place in a highly competitive hydrogen-bonding solvent, such as dimethyl sulfoxide.30 Because thiourea has a higher acidity than urea, it can generate stronger interactions with anions and form a more stable complex than urea.31 On the other hand, NaOH/ thiourea shows a stronger dissolving capacity and a higher dissolution temperature compared to those of NaOH/urea.32 It is reasonable that the stronger interactions in cellulose/NaOH/ thiourea may lead to the dissolution of cellulose at an even higher temperature. Herein, cellulose was successfully dissolved rapidly in a novel NaOH/thiourea aqueous solution at 8 °C. The dissolution mechanism of cellulose in the novel solvent system and the solution properties of cellulose as well as regenerated cellulose microspheres (CMs) were explored in detail. Moreover, the intermolecular interactions in the NaOH/thiourea as well as the role of thiourea in the dissolution of cellulose in the NaOH aqueous solution were studied. We could be more acquainted systematically with the dissolution behaviors of cellulose and propose a reasonable model to elucidate its essence in the NaOH/thiourea aqueous systems. This work provides important information to further understand cellulose dissolution, especially in the alkali system, and lays a beneficial theoretical foundation for the dissolution of cellulose in aqueous systems.

Sa = [w1/(w1 + w2)] × 100%

(1)

where w1 is the weight of the dissolved cellulose and w2 is the weight of the insoluble cellulose. Fabrication of CMs. To clarify the self-assembly behavior of the cellulose chains in the metastable solution, the CMs were prepared by the sol−gel transition method as follows. Cellulose (C-2, 4g) was dispersed in 96 g of Solvent-2 at 8 °C under vigorous stirring for 5 min to obtain a transparent cellulose solution. This solution was degassed by centrifugation at 7200 rpm for 20 min at 15 °C. A well-mixed suspension containing 300 mL of isooctane and 10 g of Span 85 was dispersed in a reactor. The resulting suspension was stirred at 1000 rpm for 50 min and then the cellulose solution was poured into the suspension within 10 min. Stirring was continued for 1 h at the same stirring speed at 25 °C. A solution containing Tween 85 in 10 mL of isooctane was then added to the emulsion and stirred at the same speed for another 1 h to produce stable emulsion droplets. Subsequently, an 80 °C water bath was applied to the emulsion for 2 min to regenerate the CMs. Dilute hydrochloric acid (10%) was then added to the resultant suspension until the pH reached 7. After removing the isooctane, the obtained CMs were washed with deionized water and then with ethanol successively three times to remove the residual isooctane, Tween 85, and Span 85. Finally, a solvent-exchange treatment with t-BuOH was performed to maintain the porous structure of the CMs. CMs containing tBuOH were frozen by liquid nitrogen and subjected to a freezedryer for characterization. Characterization. The optical transmittance (Tr) of the cellulose solution was measured on a UV−vis spectrometer (UV-6; Shanghai Meipuda instrument Co., Ltd., Shanghai, China) at a wavelength of 600 nm. 15N and 13C NMR measurements of the cellulose solution and the NaOH/ thiourea solvent systems were carried out on a Bruker AVANCE 600 NMR spectrometer at a settled temperature range of 270−298 K (−3 to 25 °C). Deuterium oxide was used as a lock signal. 13C NMR chemical shifts were externally referenced to acetone at 206 ppm. Nitromethane (CH3NO2) was used as the external chemical shift reference at 381 ppm for 15 N NMR experiments. The stability of the cellulose solution was measured on an ARES-RFS III rheometer (TA Instruments). A double-concentric cylinder geometry with a gap of 2 mm was used to determine the dynamic viscoelastic parameters. To further reveal the interactions between NaOH and

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EXPERIMENTAL SECTION Materials. Two types of cotton linter pulps as cellulose samples, coded as C-1 and C-2, were provided by Hubei Chemical Fiber Group Ltd. (Xiangyang, China), whose viscosity-average molecular weights were 4.8 × 104 and 7.6 × 104, respectively. Commercially available analytical-grade NaOH, urea, and thiourea (Shanghai Chemical Reagent Co. Ltd., China) were used without further purification. D2O (purity, 99.9%) for NMR was purchased from Cambridge Isotope Laboratories, Inc., and thiourea-15N with 98 atom % 15N for NMR was purchased from Sigma-Aldrich. Sample Preparation. As shown in the previous work, approximately 8−10 wt % NaOH aqueous solutions have the greatest ability to swell and partly dissolve cellulose.33 To search for the optimum composition of NaOH and thiourea in aqueous solution, 6−12 wt % NaOH aqueous solutions with a desired amount of thiourea were prepared. To study the role of thiourea, the weight ratio of NaOH to H2O was fixed, and only the concentration of thiourea was varied in the present work. As a result, 5 g of thiourea was added to 100 g of 10 wt % NaOH to obtain 105 g of 9.5 wt % NaOH/4.8 wt % thiourea aqueous solution (mole ratio of NaOH/thiourea/H2O, 1:0.26:20), which is coded as Solvent-1; 8 g of thiourea was added to 100 g of 10 wt % NaOH to achieve 108 g of 9.3 wt % 1794


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The Journal of Physical Chemistry B (thiourea) urea, a series of NaOH/(thiourea) urea solutions with 8 wt % (thiourea) urea and 0−10 wt % NaOH were prepared. The Raman spectra of NaOH/(thiourea) urea solutions were recorded on a LabRAM ARAMIS Raman Microscope (HORIBA Jobin Yvon, France) with an Nd/YAG laser (532 nm) at room temperature. Scans were taken over an extended range (400−4000 cm−1), and the exposure time was 120 s. To understand the cellulose aggregation behavior, dynamic light scatterings (DLSs) was used to characterize the molecular weight and hydrodynamic radius (⟨Rh⟩z) of cellulose (C-2) dilute solution in Solvent-2 at different temperatures. A commercial light-scattering spectrometer (ALV/SP-125; ALV, Germany) equipped with an ALV-5000/E multidigital time correlator and a He−Ne laser (at 632.8 nm) was used. All of the diluted cellulose solutions with concentration ranging from 0.02 to 1.0 mg/mL were made optically clean by filtration through 0.45 μm Millipore filters (NYL, 13 mm syringe filter; Whatman, Inc.). TEM images were obtained by using a JEM2010 (HT) transmission electron microscope (JEOL TEM, Japan). For the TEM observation, the samples were prepared by casting the diluted chitin solution onto a holey carbon film, which was supported on a copper grid. The specimen was dried in air under ambient conditions and imaged on a TEM at an accelerating voltage of 200 kV. A dilute solution of C-2 with a concentration of 1.0 × 10−4 g mL−1 was used here. For cryoTEM, the samples were prepared in a controlled-environment vitrification system at 277 K. The cellulose/NaOH/thiourea aqueous solution with a cellulose mass fraction of 5 mg/mL was dropped onto a carbon-coated copper grid. It was blotted with two pieces of filter paper, leading to the formation of thin films suspended on the mesh holes. The sample was quickly plunged into liquid nitrogen. The vitrified samples were stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and then observed with a JEM2200FS TEM (200 kV). The images were recorded on a Gatan Utrascan 894 CDD. The scanning electron microscopy (SEM) images of the CMs were observed by field emission SEM (FESEM, Zeiss, SIGMA) at an accelerating voltage of 5 kV. The samples were sputtered with gold before observation.

Figure 1. (a) Solubility of cellulose (1 wt %) in NaOH aqueous solution (the weight ratio of NaOH to water is 10:90) with different content of thiourea stirred at 25 °C for 1 h. (b) Photograph of cellulose solution dissolved in Solvent-2 at 8 °C. (c) 13C NMR spectra of cellulose dissolved in 10% NaOH (precooled to −10 °C), Solvent-1 (precooled to −5 °C), and Solvent-2 (precooled to 8 °C).

precooled to 8 °C, as shown in Figure 1b. The results indicated that the cellulose dissolution temperature was increased from −5 °C in 9.5 wt % NaOH/4.5 wt % thiourea34 to 8 °C by simply changing the composition of the solvent. To clarify the dissolution of cellulose in Solvent-2 at 8 °C, 13 C NMR experiments of cellulose in 10 wt % NaOH/D2O precooled to −10 °C, 9.5 wt % NaOH/4.8 wt % thiourea/D2O (Solvent-1) precooled to −5 °C, and 9.3 wt % NaOH/7.4 wt % thiourea/D2O (Solvent-2) precooled to 8 °C were performed. Six sharp signals of each cellulose solution are observed in Figure 1c, and the corresponding chemical shifts of the cellulose are summarized in Table S1. The chemical shifts of the cellulose dissolved in Solvent-1 at −5 °C and Solvent-2 at 8 °C were nearly consistent with those in 10 wt % NaOH at −10 °C, indicating that thiourea hardly interacted with the cellulose. The results also revealed that Solvent-2 at 8 °C is a direct cellulose solvent, no chemical reaction occurred, and the increase of the thiourea content in NaOH could increase the dissolution temperature of the cellulose. The stability of the cellulose solution is an important factor influencing its applications, especially for the fabrication of the regenerated material. To compare the stability of the cellulose solutions dissolved in NaOH/thiourea at different temperatures, the gelation behaviors were investigated, as shown in Figure 2. The temperature at the intersection point of the storage modulus (G′) and the loss modulus (G″) was regarded as the gelation temperature of the cellulose solution while heating. The gelation temperature increased from 41.1 °C in Solvent-1 to 45.1 °C in Solvent-2 with increasing thiourea content. This result revealed that increasing the thiourea

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RESULTS AND DISCUSSION Dissolution of Cellulose in NaOH/Thiourea at 8 °C. To optimize the concentration of NaOH for cellulose dissolution, 6−12 wt % NaOH aqueous solutions with different amounts of thiourea were prepared. With regard to the transmittance of the cellulose solutions dissolved at 25 °C (Figure S1), the NaOH aqueous solution in which the weight ratio of NaOH to water was 10:90 in the presence of thiourea is preferable over the others. Therefore, in the following part, the weight ratio of NaOH to water was fixed at 10:90 and applied for cellulose dissolution. Moreover, the transmittance of the resultant cellulose solutions increased with the thiourea content, revealing that thiourea could promote the disintegration of the inter- and intramolecular hydrogen bonds of cellulose in NaOH solution and thus enhance the dissolution of cellulose. As shown in Figure 1a, the solubility of cellulose (1 wt %) in NaOH aqueous solution (the weight ratio of NaOH to water is 10:90) increased with increasing content of thiourea stirred at 25 °C for 1 h, and the value of Sa of C-1 in Solvent-3 is the highest (89%), confirming that cellulose can be dissolved in NaOH/thiourea at a relatively high temperature. Moreover, 4 wt % C-2 was proved to be completely dissolved in Solvent-2 1795


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Figure 2. Temperature dependence of the storage modulus (G′) and loss modulus (G″) for 4 wt % cellulose dissolved in (a) Solvent-1 at −5 °C and (b) Solvent-2 at 8 °C.

Figure 3. 15N NMR spectra of (a) 8% thiourea, Solvent-2, and Solution-2 aqueous solutions and (b) 5% thiourea, Solvent-1, and Solution-1 aqueous solutions. (c) Raman spectra of 10 wt % NaOH/8 wt % thiourea, 5 wt % NaOH/8 wt % thiourea, and 8 wt % thiourea aqueous solutions. (d) Optimized structure of the NaOH/thiourea water cluster by the M06-2X/6-311G(d,p) theoretical method.

and their cellulose solutions, demonstrating the formation of a complex in the solutions. The chemical shifting of the 15N NMR spectra of thiourea in Solvent-1 and Solvent-2 to low field by 13.69 and 12.89 ppm compared to that in 5% thiourea and 8% thiourea, respectively, indicates that strong interactions existed between thiourea and NaOH in the hydrogen-bonding solvent of NaOH/thiourea/H2O. It has been reported that the hydrogen atoms of thiourea acted as hydrogen-bond donors rather than hydrogen-bond acceptors in solvents.31 The large low-field shift of the 15N NMR signal could possibly be attributed to the formation of substantial complexes between the OH− anions of NaOH and thiourea by hydrogen bonding in the NaOH/thiourea. A hydrogen bond associated with a

content had a favorable effect on preventing the self-association of the hydrogen-bonded junction between the cellulose chains, leading to the increase in the stability of the cellulose solutions. Interactions between Cellulose and NaOH/Thiourea. The presence of urea or thiourea dramatically improves the solubility of the resultant cellulose solution in NaOH.20,21 To clarify the role of thiourea on the cellulose dissolution, 15N NMR spectra of 5% thiourea, 8% thiourea, Solvent-1, Solvent-2, Solution-1, and Solution-2 were measured, as shown in Figure 3a,b. The chemical shifts of the 15N NMR spectra of all samples are summarized in Table S2. The 15N NMR spectra of both 5 and 8% thiourea solutions exhibited a broad signal, whereas sharp signals appeared in the spectra of Solevent-1, Solvent-2, 1796


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Figure 4. (a) Hydrodynamic diameter (Rh) distribution of cellulose dissolved in Solvent-2 at 8 °C in the temperature range of 12−65 °C. (b) TEM image of the cellulose solution with a concentration of 1 mg/mL dissolved in Solvent-2; (c, d) schemes of cellulose chains and the cellulose aggregate in NaOH/thiourea aqueous solutions.

formation of thiourea···OH− complexes through hydrogen bonding in NaOH/thiourea. Increasing the NaOH concentration to thiourea solution led to the absorption of the antisymmetrical CN stretching vibration at a higher wavenumber, which resulted from an increase in the strength of hydrogen bonding in thiourea···OH− complexes. Furthermore, a new Raman band at 1357 cm−1 appeared with the addition of NaOH, assigned to the NCN and CS stretching vibrational modes.35 The intensity of this new band increased with the content of NaOH, indicating the existence of short-range interactions between NaOH and thiourea. On the other hand, thiourea aqueous solutions with and without NaOH were also measured by a Raman spectrometer (Figure S2). The Raman spectra of the urea solutions with and without NaOH hardly changed, suggesting that no stronger interactions occurred between NaOH and CN or NH2 of urea than those between NaOH and thiourea. This result also supported the observation of only a small change between the 15N NMR signals of 7 wt % NaOH/12 wt % urea and 12 wt % urea aqueous solutions,23 compared to that of the thiourea solvent system. To further reveal the interaction among the small molecules in NaOH/thiourea, the M06-2X/6-311++G(d,p) level of theory was applied.37,38 M06-2X/6-311++G(d,p) was implemented in the Gaussian 03 package.39 On the basis of the results of the 15N NMR and Raman spectra, NaOH(H2O)5· thiourea, NaOH(H2O)6·thiourea, NaOH(H2O) 7·thiourea, NaOH(H2O)8·thiourea, and NaOH(H2O)9·thiourea were designed and optimized, as shown in Figure S3. The successive binding energies were calculated to be −8.8, −11.9, −6.6, and −7.9 kJ/mol for NaOH(H2O)6·thiourea, NaOH(H2O)7·thio-

proton attached to nitrogen reduced the electron cloud density of nitrogen and thus caused the nitrogen lone pairs to be more delocalized and the nitrogen atoms to be more positive, leading to a C−N double-bond character on thiourea. In our previous work, the addition of NaOH to a 15N-urea/D2O solution, however, caused only a small shift of 0.49 ppm to the low field,23 which was much smaller than the shift of 13.69 or 12.89 in the present work. This could be explained by the fact that thiourea was more acidic than urea, and it exhibited stronger interactions with OH− anions and thus formed more stable complexes than urea.30 Furthermore, the 15N chemical shifts of thiourea in Solvent-2 shifted to high field by 0.8 ppm, compared to that of Solvent-1. In our findings, the mole ratio of NaOH to thiourea in Solvent-1 was larger than that in Solvent-2, leading to the interaction of more OH− anions with thiourea to form stronger hydrogen bonds in Solvent-1, which in turn led to a slight decrease in the electron delocalization of the nitrogen lone pair and thus the low-field shift of the 15N NMR signal. Raman spectroscopy has been widely exploited to reveal the association interaction of thiourea and salts.35,36 In the present article, Raman spectra of 8 wt % thiourea aqueous solutions with different concentrations of NaOH from 0 to 10 wt % are displayed in Figure 3c. In the thiourea aqueous solution, the Raman band at 1486 cm−1 was assigned to the antisymmetrical CN stretching vibration. With the addition of 5 and 10 wt % NaOH to the thiourea solution, such vibration shifted to a higher wavenumber from 1486 to 1498 and 1509 cm−1, respectively, indicating that the CN bonds became shorter and possessed partial CN double-bond character because of the 1797


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dilute solution increased. Moreover, ⟨Rh⟩ of the single chains in the NaOH/thiourea decreased slightly with an increase in temperature. This could be explained by the fact that cellulose chains with a high molecular weight preferred to aggregate,40 resulting in cellulose chains with a low molecular weight mainly remaining as single chains (low ⟨Rh⟩) in the solution, in which aggregates existed as the predominant species of cellulose. It was worth noting that, to avoid aggregation, extremely dilute solutions of cellulose were prepared for DLS; however, two peaks were still observed in the extremely dilute solution with a concentration lower than 0.01 mg/mL, indicating that the cellulose solution was metastable because of the dynamic intermolecular interactions among NaOH, thiourea, and cellulose in the solution. To clarify the morphology of the cellulose chains as well as their aggregates in NaOH/thiourea, TEM observation of the dilute Solution-2 was carried out, as shown in Figure 4b. The extended cellulose chains and their aggregates as nanofibers were observed, further confirming that the cellulose as extended wormlike chains and their aggregates coexisted in the dilute cellulose solution. The extended cellulose−NaOH−thiourea complex was about 200−300 nm in length and 4.8 ± 0.6 nm in diameter, as evaluated by the TEM images, similar to that in the NaOH/urea aqueous solution (4.7 nm).23 The results suggested that cellulose in the NaOH/urea and NaOH/ thiourea aqueous solutions exhibited an analogous intermolecular interaction as well as a similar dissolution mechanism. However, cellulose chains in the NaOH/thiourea system more easily self-aggregated in parallel to form compact bundles. As expected, the extended wormlike cellulose chains and their aggregates as nanofibers with a mean diameter of 24 nm were also observed in the cryo-TEM images (Figure S4). Because of the rapid deep-freezing procedure in liquid nitrogen during sample preparation for the observation of the cryo-TEM images, the cellulose−NaOH−thiourea complexes and their nanofibers could maintain their original morphology in solution. These results indicated that the extended cellulose chains are easily self-assembled in parallel to form nanofibers in NaOH/thiourea because of their metastable properties. In view of the above results, schemes to describe the cellulose complexes associated with NaOH and thiourea as well as the formation of nanofibers composed of stiff cellulose chains are shown in Figure 4c,d. A large number of hydroxyl groups on the cellulose were attached with OH− of the thiourea···OH− complexes in solvent to form complexes, leading to cellulose dissolution (Figure 4c,d). Furthermore, the dynamic intermolecular interaction between cellulose and thiourea···OH− complexes caused a metastable feature, leading to the detachment of partial thiourea···OH− complexes as well as the aggregation of the exposed cellulose chains in a parallel pattern to form nanofibers, as shown in Figure 4c. Construction of Nanofibrous CMs. It is widely acknowledged that materials with three-dimensional nanotopographical structures have broad applications in tissue engineering,41,42 green catalysis,43 and so on. The “bottom-up” (from microscopic to macroscopic) fabrication pathway was utilized for the reconstruction based on natural polymers, such as cellulose,3 chitosan,44 chitin,41 and collagen.45 Usually, the cellulose chains in the metastable solution could easily aggregate due to the existence of fluctuations, such as elevating temperature; thus, it was possible to fabricate nanofibrous cellulose materials by inducing with heat. In the cellulose solution dissolved at a relatively high temperature, the thiourea−NaOH sheath around

urea, NaOH(H2O)8·thiourea, and NaOH(H2O)9·thiourea, respectively. The results illustrated that the NaOH(H2O)7· thiourea cluster is a preferable cage and was more stable than the others because of the highest successive binding energy (Table S3). In this cage structure (Figure 3d), the proton on the amino group of thiourea was easily transferred to the OH− anions, and the C−N bond possessed partial double-bond character, which is consistent with the results of 15N NMR and Raman spectra, demonstrating the possibility of the formation of a potential cluster structure composed of solvent molecules by hydrogen bonding. Moreover, when cellulose was added, as shown in Figure 3a,b, the 15N chemical shifts of both Solution-1 and Solution-2 moved to high field but with a tiny shift of about 0.2 ppm, indicating that the NaOH/thiourea complexes interacted with the cellulose and probably resulted in the cellulose−NaOH− thiourea−water complexes. This tiny shift was attributed to the interaction between OH− anions and the hydroxyl groups of the cellulose, which slightly reduced the hydrogen-bonding interaction between NaOH and thiourea, leading to a slight decrease of the electron delocalization of the nitrogen lone pair. As it is well known, the addition of thiourea or urea dramatically improved the solubility of cellulose in NaOH aqueous solution, and 9.5 wt % NaOH/4.5 wt % thiourea demonstrated a higher dissolution capacity against cellulose than 7 wt % NaOH/ 12 wt % urea, possibly because the strength of the hydrogen bonds in thiourea···OH− complexes is higher than that in urea···OH− complexes revealed by 15N NMR, suggesting that strong hydrogen bonding with NaOH is important for the dissolution of cellulose. By continuously increasing the content of thiourea in NaOH aqueous solution, the solubility of cellulose could be further improved (Figure 1a) and the dissolution temperature of cellulose can be increased as well (Figure 1c). However, the hydrogen bonding in the thiourea···OH− complexes in Solvent2 is weaker than that in Solvent-1. On the other hand, it is notable that the number of thiourea···OH− complexes in Solvent-2 is much higher than that in Solvent-1. The key points of the cellulose dissolution were the cleavage of the intermolecular hydrogen bonds of the cellulose and the deceleration of the re-formation of intermolecular hydrogen bonds among the dispersed cellulose chains. The increase in the amount of thiourea···OH− complexes could significantly reduce the probability of the self-association of intermolecular hydrogen bonds among the cellulose chains to improve the cellulose solubility. Solvent-2 displayed better solubility of cellulose than Solvent-1, confirming that the amount of thiourea···OH− complexes played a dominant role. Solution Behaviors of Cellulose in NaOH/Thiourea Dissolved at 8 °C. After the dissolution of cellulose at 8 °C in Solvent-2, understanding its solution behavior is essential for its successful application. A DLS measurement of the dilute cellulose solution was performed. As shown in Figure 4a, there were two peaks in the hydrodynamic radii (⟨Rh⟩) distribution of the cellulose solution in NaOH/thiourea at 12 °C. The peak with a smaller ⟨Rh⟩ represented the individual cellulose chains, whereas that with a larger ⟨Rh⟩ could be attributed to their aggregates. The apparent average, ⟨Rh⟩app, obtained from the cumulative analyses was ∼22 nm for the individual chains and ∼210 nm for the aggregates in the cellulose solution. With increasing temperature, the cellulose aggregates significantly peak-shifted from 210 nm at 45 °C to 410 nm at 65 °C, suggesting that the content of the aggregates in the cellulose 1798


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Figure 5. (a, b) SEM images of nanofibrous microspheres consisting of cellulose nanofibers. (c) Magnified image of image b (scale bar = 200 nm).

the cellulose was easily destroyed, and the stiff cellulose chains quickly self-aggregated in parallel (with the largest contact area) through the hydrogen bonding to form the nanofibers. In general, the stiff polymer chains tend to self-assemble in a parallel manner to nanofibers because of the easy arrangement.46−48 Herein, the representative nanofibrous CMs were constructed from a 5 wt % cellulose solution in 9.3 wt % NaOH/7.4 wt % thiourea dissolved at 8 °C by inducing with heat. The SEM images of the nanofibrous CMs are illustrated in Figure 5. The microspheres consisted of cellulose nanofibers with a mean diameter of 23 nm, which was consistent with that from cryo-TEM images, and the size of the microspheres was in the range of 20−180 μm. The microspheres exhibited a uniform architecture and a well-distributed apparent porous structure (pore size, about 20−200 nm), and no pore walls were observed. This could be explained by the fact that an increase in the number of relatively stable thiourea···OH complexes in Solution-2 led to an increase in the stability of the cellulose chains in the solution, and that these complexes are well aligned along the axial direction of the cellulose chains rather than random aggregation. Thus, the cellulose nanofibers, as the predominant species of aggregates, were formed via heating, leading to the formation of nanofibrous CMs. This work would open a new pathway for the construction of cellulose-based nanomaterials from the metastable solution.

from the cellulose metastable solution dissolved in NaOH/ thiourea aqueous solvent at 8 °C.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b10829. Transmittance of cellulose solutions, Raman spectra of NaOH/urea solutions, optimized structures of NaOH/ thiourea clusters, cryo-TEM images, and 13C and 15N NMR data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +86-27-87219274 (A.L.). *E-mail: [email protected] (L.Z.). ORCID

Lina Zhang: 0000-0003-3890-8690 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major Program of National Natural Science Foundation of China (21334005), the Major International (Regional) Jo in t Resear ch Pr oject (21620102004), and the National Natural Science Foundation of China (51203122 and 51573143). The authors gratefully acknowledge Prof. Zhibo Li of Qingdao University of Science and Technology for his help with cryo-TEM measurements.

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CONCLUSIONS Cellulose was successfully dissolved in 9.3 wt % NaOH/7.4 wt % thiourea aqueous solution at 8 °C. In this cellulose solution, OH− anions of the NaOH could directly associate with the amino groups of the thiourea to form relatively stable thiourea···OH− complexes through strong hydrogen bonds. The NaOH(H2O)7·thiourea cluster was the most stable structure determined by density functional theory (DFT). The strength of the hydrogen bonds in the thiourea···OH− complexes was much higher than those in urea···OH−, leading to a higher solubility of cellulose in NaOH/thiourea than that in NaOH/urea. The cellulose solution in the NaOH/thiourea aqueous solvent at 8 °C was a metastable solution, and the extended wormlike cellulose chains and their aggregates coexisted in the cellulose dilute solution, supported by the results of DLS and TEM. The NaOH−thiourea sheath around the cellulose complexes could be broken at elevated temperatures or concentrations to aggregate in parallel to form nanofibers. As a result, the cellulose nanofibers as the predominant species of aggregates were formed via heating, leading to the formation of nanofibrous CMs. This work opened a new pathway for the construction of nanomaterials

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